[Gimp-user] jpeg loss on multiple export

At the risk of committing a mailing list etiquette fopah...
I'm reposting as I got no response and I am hoping one of the developers can actually shed light on this:

On 5/3/2012 8:15 AM, Jay Smith wrote:

Jonathan, I hope that you realized that when editing a JPG, repeatedly
saving/exporting to JPG (your step 4) reduces the quality (actually
compresses / deletes data).

GIMP doesn't reload the file after exporting, does it?
If not, then the image in the GIMP's buffer should be the same high quality image it was prior to the export. Given that, as long as you don't exit the GIMP or reload the image from the exported file, multiple exports to a lossy format shouldn't cause loss of quality.

For example, if you do an export to jpeg and compress down to 15%, with the preview option set you can clearly see the loss of quality in the written image. However, when the export is complete, the image in the buffer reverts to the high quality it was prior to the export.

This implies multiple exports to a lossy format should *not* result in loss of image quality. However, *re-loading* an image previously saved to a lossy format and then exporting again will result in degradation.

Is there something I'm missing?


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